With eight 2x4s, some screws, and some string, you can make a tomato trellis that will grow 30 or 40 tomato plants 8 feet high. This layout is designed to be as simple, cheap, and easy as possible. Eight 2x4s will run you around $20.
You'll need a hand saw or rotary saw, a table saw, a measuring tape, and a drill.
I didn't have a table saw, so I cut the boards at Techshop before doing the assembly at home.
"I made it at Techshop"
Step 1: Rip the 2x4s
I didn't get a picture of this step, but it's an easy one. You want to turn eight 2x4s into sixteen 2x2s. Use a table saw to cut each 2x4 in half length-wise.
Remember, 2x4s are usually more like 1.5x3.5s, so set the table saw guard to about 1.25" to get a cut down the middle.
Step 2: Lay Out the Boards
Here you see six of the seven 2x2s required for each side of the trellis. The seventh will go horizontally to brace the bottom of the two diagonals.
Technically you could just screw everything together here without making any more cuts. In the next step I show how I cut a few angles for neatness sake, but this is completely optional.
Step 3: Cut Some Angles and Screw Everything Together
For the two angled boards to meet in the middle, you want the top to be about 135 3/4" (11 feet, 3 3/4") long
The two angled boards will meet the two vertical boards at 67 7/8" (5 feet, 7 7/8") from the top.
Put the boards together however you like. They're long and flexible enough that even if there's a twist (like there is in my bottom horizontal board), it will turn out ok.
TIP: assemble each joint with only one screw at first (so things are free to rotate). This lets you swivel the pieces to line up correctly without having to use a square for your angles. Once everything is "pinned" in place, you can follow up with some more screws.
(calculation: each angled board is 8 feet (96") long and at a 45 degree angle. That means the short sides of the triangle are 96" * sin(45 degrees), or 67 7/8". multiply that by 2 and you get 135 3/4" for the top)
Step 4: Screw the Tops of the Frame Pieces Together, and Hang Strings
Now screw the tops of the two frame pieces together and brace with a crosspiece on each side. Paint everything if you like, and hang strings.
This video (not by me) shows how to attach your tomato plants to the strings.